Heat broiler. In a medium saucepan, combine lentils and 3 C water. Bring to a boil over high heat. Reduce heat to low, cover and simmer 5 minutes. Add onion, carrot, and celery. Cover and continue cooking until lentils are tender, about 20 to 25 minutes. Drain, reserving liquid.

In a small bowl, whisk cooking liquid with vinegar, oil, mustard, and garlic. Toss lentils with half of dressing.

Season salmon with salt and pepper. Coat a baking pan with nonstick spray. Place salmon, skin side down, on prepared pan. Broil until salmon is cooked through, about 8 to 10 minutes. Divide lentil mixture among 4 plates. Flake salmon on top of the lentils and drizzle remaining dressing over all.